Synthesis and structures of dinuclear palladium complexes with 1,3-benzimidazolidine-2-thione and 1,3-imidazoline-2-thione

The synthesis and structures of dinuclear palladium complexes with 1,3-benzimidazolidine-2-thione and 1,3-imidazoline-2-thione are reported.


Chemical context
The coordination chemistry of N,S-donor heterocyclic-2thione ligands has been in focus for the past four decades, describing synthetic methods, bonding and structures of metal complexes (Raper, 1985(Raper, , 1994(Raper, , 1996(Raper, , 1997García-Vá zquez et al., 1999;Akrivos, 2001), analytical chemistry (Koch, 2001), charge-transfer complexes (Serpe et al., 2008) and anion receptors (Bondy & Loeb, 2003). A recent survey revealed that the reactions of heterocyclic-2-thiones with group 10-12 metals (Ni-Pt, Cu-Au, Zn-Hg; Lobana, 2021) have led not only to the formation of a variety coordination compounds, but have also displayed other aspects of chemical reactivity. For instance, some reactions of heterocyclic thiones involved copper-mediated activation, and rupture of C-S (thione) bonds followed by their transformations to other forms of thio-ligands, bonded to the copper metal. Further, there has been an upsurge in explorations of the bio-activity and biosafe potential of coordination compounds, as antimicrobial and anticancer agents (Lobana, 2021).

Structural commentary
The reaction of PdCl 2 (PPh 3 ) 2 with bzimtH 2 in a 1:2 molar ratio in the presence of Et 3 N base was designed to form [Pd( 1 S-bzimtH) 2 (PPh 3 ) 2 ] after removal of both halogens as [Et 3 NH] + Cl À . However, the X-ray crystal structure of the product revealed the formation of the unexpected dinuclear compound [Pd 2 (-N,S-bzimtH) 2 (CN) 2 (PPh 3 ) 2 ] (1). Another thio-ligand, imtH 2 yielded a similar dinuclear compound, [Pd 2 (-N,S-imtH) 2 -(CN) 2 (PPh 3 ) 2 ] (2). In both these compounds, the anionic bzimtH À and imtH À ligands coordinate through N,S donor atoms in a bridging mode, covering four coordination sites of two metal centers, and other two sites are occupied by two PPh 3 ligand molecules. Finally, the remaining two sites of two metal centers are occupied by cyano groups, abstracted by the metals from the solvent during reaction.
Compound 1 crystallizes in the monoclinic space group C2/ c, and compound 2 in the monoclinic space group, P2 1 /c. Selected bond distances and bond angles are given in Tables 1 and 2, respectively. The molecular structure of compound 1 is shown in Fig. 1, while that of compound 2 is shown in Fig. 2 (leaving out the acetonitrile solvent molecules). Considering first the structure of compound 1, here only half of the molecule is unique as the molecule lies on a crystallographic twofold axis. In 1, the Pd metal atom is bonded to one P, one S, one N and one C atoms with the respective bond distances as follows: Pd-P = 2.2861 (6), Pd-S = 2.3547 (6), Pd-N = 2.0545 (17), and Pd-C = 1.959 (2)  Diagram of 2 showing the atom labeling and the strong intramolecularinteractions involving both the thione rings and adjacent phenyl rings from the triphenylphosphine ligand. Atomic displacement parameters are at the 30% probability level.

Figure 1
Diagram of 1 showing the atom labeling for unique atoms (the molecule lies of a twofold axis; symmetry operation to generate the rest of the molecule is Àx, y, 1 2 À z) and the strong intramolecularinteractions involving both the thione rings and adjacent phenyl rings from the triphenylphosphine ligand. Atomic displacement parameters are at the 30% probability level. P-Pd-S and N-Pd-C, of 172.26 (2) and 178.31 (8) , as well as the cis bond angles in the range 84.93 (6)-94.24 (5) , reveal the distorted square-planar geometry of each metal center. One of the major factors in the conformation adopted by the molecule is the stronginteraction between the thione moieties [CgÁ Á ÁCg, 3.1905 (12) Å ], as seen in Fig. 1. In addition, there is also ainteraction between the thione moieties and an adjacent phenyl ring from the triphenylphosphine ligand [CgÁ Á ÁCg = 3.3560 (9) Å with a slippage of 1.408 Å ].
The coordination pattern of compound 2 is similar to that of 1. Nevertheless, there are minor differences in the bond distances and angles pertaining to the two metal centers of compound 2 (Fig. 2). Thus, the respective Pd-P, Pd-S, Pd-N and Pd-C bond distances of 2 are 2.2914 (5), 2.3541 (5), 2.0345 (17) and 1.957 (2) Å (Pd1 metal center), and 2.2984 (5), 2.3542 (5), 2.0345 (17) and 1.943 (2) Å (Pd2 metal center). For both metal centers, the trans bond angles [P-Pd-S and N-Pd-C = 173.86 (2)-179.31 (8) ] and the adjacent bond angles [86.54 (7)-92.80 (4) ] are similar to those of compound 1. These bond angles again reveal the distorted square-planar geometry of each metal center of compound 2. The various bond distances described above are normal and none unusual. Compound 1 has carbon-sulfur (C-S) bond distance of 1.728 (2), while in compound 2 it is 1.734 (2) Å . These distances are in between single (1.81 Å ) and double-bond (1.68 Å ) C-S distances (Huheey et al., 1993). It shows a weakening of the C-S bond as a result of S to Pd coordination. The C N distance of the coordinated cyano group is 1.127 (3) in compound 1 and 1.143 (3) /1.148 (3) Å in compound 2. These distances are less than the expected C N double bond (1.28 Å ) and are close to the C N triple bond distance (1.15 Å ; Huheey et al., 1993). The structure of 2 contains partially occupied acetonitrile solvent molecules with occupancies of 0.33 and 0.25. As in the case of 1, in 2 one of the major factors in the conformation adopted by the molecule is the stronginteraction between the thione moieties [CgÁ Á ÁCg = 3.3559 (12) Å ], as seen in Fig. 2. In addition, there is also ainteraction between each of the thione moieties and an adjacent phenyl ring from the triphenylphosphine ligand [CgÁ Á ÁCg distances of 3.3065 (8) Å and 3.3218 (8), respectively, with a slippage for the latter of 1.154 Å ].
The IR spectrum of the bzimtH 2 ligand showed a (N-H) band at 3113 (m), and in compound 1, this band appeared at a lower energy, 3055 (m) cm À1 . The ligand showed a diagnostic (C S) band at 1179 cm À1 , which shifted to (C S), 1033(s) cm À1 , owing to the change of neutral bzimtH 2 ligand to the bzimtH À anionic form, coordinating through N,S donor atoms. The PPh 3 ligand showed its characteristic (P-C Ph ) band at 1097(s) cm À1 in compound 1. A band at 1734 cm À1 was assigned to the coordinated cyano group. The IR spectroscopic bands of compound 2 are similarly assigned: (N-H), 3050 (m), (C S), 1020 (m), (P-C Ph ), 1105 (s) and (C N), 1740(s) cm À1 .
In conclusion, the chemistry of heterocyclic-2-thiones remains enigmatic, probably due to the angular flexibility at sulfur, and also due to the short bite angle of the N,S-donor set in case it chelates with the formation of four-membered rings. This leads to a greater tendency of these thio-ligands in anionic forms to adopt bridging modes, noted as for example in dinuclear complexes (Raper, 1997;Lobana, 2021). Benz-1,3-imidazoline-2-thione (bzimtH 2 ) has formed an N,S-bonded symmetrically bridged dinuclear compound, and so is the case with 1,3-imidazolidine-2-thione, and these are analogous to literature reports (Yamamoto et al., 1991;Yap & Jensen, 1992).

Supramolecular features
In the packing of 1 and 2 there are similar trends in both hydrogen-bond patterns and intramolecular interactions. In both structures, there are strong intramolecularinteractions involving the thione moiety and adjacent phenyl rings from the triphenylphosphine ligand as discussed above. Both 1 and 2 have a similar hydrogen-bonding pattern (numerical details in Tables 3 and 4 Table 1 Selected geometric parameters (Å , ) for 1.   Hydrogen-bond geometry (Å , ) for 1.
N-H group of the thione moiety forms an intermolecular hydrogen bond with an adjacent N atom from the coordinated cyanide anion and these form C 1 1 (7) chains (Etter et al., 1990) in the [110] and [110] directions. In addition, in 2 there are also C-HÁ Á ÁN interactions between the imidazoline rings and the partially occupied acetonitrile N atoms and this is shown in Fig. 5.
Preparation    Hydrogen atoms not involved in hydrogen bonding are omitted for clarity. Symmetry operation to generate the rest of the molecule is Àx, y, to it was added triphenylphosphine (0.148 g, 0.564 mol). The contents were refluxed for 1 h and the yellow complex formed was filtered and dried in vacuo, m.p. 551-553 K Preparation of 1 To a solution of PdCl 2 (PPh 3 ) 2 (0.030 g, 0.043 mmol) in 10 mL of CH 3 CN, was added solid bzimtH 2 (0.013 g, 0.086 mmol) followed by the addition of Et 3 N base (0.5 mL). The solution became yellowish orange and was refluxed for 6 h. The orange compound was formed on refluxing. It was separated and dissolved in a solution of methanol (4 mL) and dichloromethane (1 mL) in a culture tube. A slow evaporation of the reaction mixture over a period of one month, resulted in the formation of orange crystals of compound 1. Yield: 0.015 g; 65%; m.p. 511-513 K. Analysis found: C, 57.71; H, 3.84; N, 7.50; C 52 H 40 N 6 P 2 Pd 2 S 2 (1087. Preparation of 2 To the solution of PdCl 2 (PPh 3 ) 2 (0.040 g, 0.060 mmol) in 10 mL of CH 3 CN, was added solid imtH 2 (0.012 g, 0.120 mmol) followed by the addition of Et 3 N base (0.5 mL). The solution became yellowish orange and was refluxed for 6 h. The orange compound was formed on refluxing and was separated. It was dissolved in a solution of methanol (4 mL) and dichloromethane (1 mL) in a culture tube. Slow evaporation of the reaction mixture over a period of one month formed yellowish-orange crystals of compound 2.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Hydrogen atoms were fixed geometrically (C-H = 0.93-0.98 Å ) with their U iso (H) = 1.2U eq (C). The structure of 2 contains partially occupied acetonitrile solvent molecules with occupancies of 0.33 and 0.25.   For both structures, molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).